1. Technical Field
This invention relates to the technology of preparing surfaces to accept sprayed coatings, and more particularly to the use of electrical discharge for roughening surfaces to promote a mechanical bond with such coatings.
2. Discussion of the Prior Art
Surfaces to be commercially thermally coated have been previously prepared by the prior art by essentially 3 types of preparation: grit blasting, high pressure water jetting, and high frequency magnetic field. Grit blasting requires a high pressure flow of a fluid medium to carry the grit to the surface to be roughened. The flow must have a high pressure, usually greater than 15 psi; it utilizes a grit medium such as silica sand, aluminum oxide, chilled iron or garnet to produce a roughened surface. After grit blasting, the surface must be cleansed of the grit and dust before coating thereover. Grit blasting for high volume on-line manufacturing of thermal sprayed coated components is accompanied by the following concerns: grit contamination of the components and process equipment; difficulty of on-line collection and containment of the grit medium; grit blasting may not allow for highly accurate dimensional control; grit blasting is a line of sight process making surface preparation of complex components difficult; and sometimes reproducibility of surface roughness is difficult to achieve. Water jetting requires even higher pressures to impact the metal surface to dimple and abrade the surface. Such technique requires very high powered pumps, each of which are expensive and lack precise controllability of roughening.
Use of a high frequency magnetic field to disrupt the oxide film on a metal surface to be cleaned may be effective, but it is not particularly useful in creating a roughened surface. Acid etching has sometimes been utilized, but is troublesome because of the noxious content of the acid.
Although not used for surface roughening, electrical discharge machining has been utilized to create smooth metal surfaces or to cut smoothly through metals. Electrical discharge machining has been known for close to 50 years. A workpiece (usually the cathode) has material removed by it by an arc struck between a tool (electrode) and the workpiece to discharge electrical current. Every discharge pulse is like a miniature lightening bolt that melts, vaporizes and removes a minuscule portion of the workpiece without mechanical contact or stress supplied by the electrode on the workpiece. A dielectric liquid is used in the gap between the material to be eroded and the electrode. The liquid serves to carry away machined particles as the electrolyte flows through the gap. Electrical discharge machining has been developed to machine (cut and shape) relatively smooth surfaces (surface roughness under 10 micro inches) in metals having electrical resistivity under 300 ohms/cm. Such electrical discharge machining is not effective in creating a rough surface that locks coatings thereon because of the character of the resultant smooth surface. More importantly, when electrical discharge machining techniques are applied to aluminum or iron substrates, the resulting surface is burned or passivated, leaving an oxide film which is not smoothly dimpled, like an orange peel texture. Such passivated surface will not allow for proper adherence of a coating deposited thereover. This necessitates that the passivated surface must be removed, which can be accomplished by further grit blasting or acid dissolution techniques, both being undesirable and adding to the cost of the preparation.